Area Forecast Discussion
Issued by NWS Chicago, IL

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669
FXUS63 KLOT 040747
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
247 AM CDT WED MAY 4 2016

.UPDATE...
838 PM CDT

THERE WERE ONLY A FEW CHANGES MADE TO THE FORECAST TONIGHT AND ALL
FAIRLY MINOR SINCE THE MAIN THEME OF THE FORECAST
CONTINUES...SHOWERS AND SOME STORMS TONIGHT WITH WINDY AND
COOLER CONDITIONS MOVING IN. THE CHANGES INCLUDED 1.) HAVING
INCREASED THE THUNDER LIKELIHOOD OVERNIGHT...2.) BUMPED UP WIND
GUSTS LATE TONIGHT AND WEDNESDAY MORNING ESPECIALLY NEAR THE LAKE
SHORE WHERE SOME GUSTS OF 40 TO 45 MPH ARE POSSIBLE...AND 3.) FOR
WEDNESDAY AFTERNOON WENT WITH MORE COVERAGE WORDING OF SHOWERS AS
CONFIDENCE IN DEVELOPMENT IS FAIRLY HIGH...ESPECIALLY FOR AREAS
EAST OF INTERSTATE 39.

WATER VAPOR IMAGERY THIS EVENING PORTRAYS A REALLY WELL-DEFINED
SHORT WAVE UPPER CIRCULATION AT THE LEADING EDGE OF A SHARPLY
DIGGING BROADER TROUGH. THE 00Z INL SOUNDING SAMPLED A 115 KT DUE
NORTHERLY JET...HIGHLY ANOMALOUS ORIENTATION AND SPEED FOR MAY.
THE SHORT WAVE HAS BEEN ON A STEADY TRACK TOWARD THE AREA WITH ITS
LEADING EDGE PRESENTLY IN CENTRAL WISCONSIN. CONVECTION AND
ASSOCIATED LIGHTNING HAS BEEN STEADY ALONG THIS FRONT EDGE AND
OFTEN WITH THESE COLD CORE WAVES KEEP THEIR LIGHTNING OVERNIGHT.
SO HAVE INCREASED THUNDER CHANCES AS STEEP MID-LEVEL LAPSE RATES
/SAMPLED ON 00Z MPX SOUNDING/ MOVE INTO THE REGION. THE OVERALL
COVERAGE OF SHOWERS AND ANTICIPATED FORCING...AS WELL AS OUTPUT
FROM CONVECTIVE ALLOWING MODELS...IS HIGH ENOUGH TO WARRANT NEAR
100 POPS BETWEEN MAINLY 11 PM AND 4 AM FROM NORTH TO SOUTH.

UPSTREAM OBSERVATIONS AT THE SURFACE ARE INDICATE A STRENGTHENING
ISALLOBARIC COUPLET AS THE LOW OF 1002 MB NEAR LA CROSSE
WISCONSIN CONTINUES TO DEEPEN. SOME UPSTREAM WINDS ARE ALREADY
OUTPERFORMING GUIDANCE AND WOULD EXPECT THAT TO CONTINUE TO BE THE
TREND IN THIS PATTERN. HAVE INCREASED GUSTS WITH TEMPORARY 30 TO
40 MPH GUSTS EXPECTED OVER MUCH OF THE AREA IMMEDIATELY BEHIND THE
FRONT AND THEN 30 TO 35 MPH GUSTS MUCH OF THE DAY WEDNESDAY.
CLOSER TO THE LAKE SHORE GIVEN THE FRICTIONLESS NORTHERLY
DIRECTION THE GUSTS SHOULD PEAK MORE 35 TO 45 MPH DURING WEDNESDAY
MORNING.

WAVES ARE LIKELY TO BUILD TO OR A LITTLE OVER 10 FT ON THE
CHICAGO SHORE BY LATER WEDNESDAY MORNING AND 12 TO 15 FT ALONG
THE INDIANA SHORE BY EARLY AFTERNOON. THIS FORECAST IS BELOW
THRESHOLDS FOR A LAKE FLOOD ADVISORY.

AS FOR FORECAST TEMPERATURES WEDNESDAY...THESE CONTINUE TO LOOK
TO BE ON TRACK WITH MID TO UPPER 40S HIGHS IN THE DOWNSTREAM PART
OF THE LAKE AND LOWER TO MID 50S ELSEWHERE.

MTF

&&

.SHORT TERM...
228 PM CDT

THROUGH WEDNESDAY...

BIG CHANGES ARE IN STORE IN THE SHORT TERM. HIGH PRESSURE IS SLOWLY
SHIFTING SOUTH AS LOW PRESSURE AND A COLD FRONT DROP SOUTH OUT OF
ONTARIO.  EXPECTING A PLEASANT AFTERNOON AND EVENING WITH WEST WINDS
AND FAIR WEATHER CUMULUS CLOUDS.  SHOWERS HAVE ALREADY FORMED AHEAD
OF THE COLD FRONT OVER THE U.P. OF MICHIGAN. REALLY LIKING THE RAP/S
TIMING SO LEANED HEAVILY IN ITS DIRECTION...WITH THE NAM BEING A
SLOWER SOLUTION.  HAVE SLIGHT CHANCE TO CHANCE POPS OUT AHEAD OF THE
MAIN FRONTAL PRECIP JUST IN CASE A FEW STRAY SHOWERS FORM THIS
EVENING. KEPT THE PREVIOUS FORECASTS IDEA OF NUMEROUS SHOWERS
WITH A FEW EMBEDDED THUNDERSTORMS. SEVERE STORMS ARE NOT EXPECTED
AS CAPE VALUES WILL BE MINIMAL AT BEST.

THE BIGGER STORY WILL BE WINDS SHIFTING OUT OF THE NORTH AND GUSTING
TO 30 MPH BEHIND THE FRONT. WINDS WILL BE EVEN GUSTIER ALONG THE
SOUTHERN END OF LAKE MICHIGAN ESPECIALLY ALONG THE INDIANA SHORE.
MUCH COOLER TEMPERATURES FOLLOW THE FRONT WITH HIGHS IN THE LOW TO
MID 50S AWAY FROM THE LAKE.  HIGHS ALONG THE LAKE WILL BE IN THE
UPPER 40S TO MAYBE 50.  THE BACK END OF THE UPPER LEVEL LOW ROTATES
THROUGH TOMORROW AND EXPECTING SCATTERED SHOWERS TO FORM.  FORECAST
SOUNDINGS SHOW VERY SHALLOW CAPE...BUT ANY CONVECTION WOULD BE VERY
LOW TOPPED.  FREEZING LEVELS ARE ALSO LOW AROUND 3000 FT SO THINKING
THE LOW CAPE AND STEEP LAPSE RATES WILL LEAD TO GRAUPEL INSTEAD OF
THUNDER.  NOT EXPECTING A TON OF RAIN...MORE ALONG THE LINES OF
DREARY CONDITIONS WITH OCNL LIGHT RAIN AND POSSIBLY GRAUPEL.

JEE

&&

.LONG TERM...
259 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

THE PERIOD WILL BEGIN ON THE CHILLY SIDE WEDNESDAY NIGHT AS THE
LINGERING EFFECTS OF THE UNSEASONABLE STRONG UPPER LOW CONTINUES
TO SHIFT OVER THE LOWER OHIO VALLEY. CLEARING SKIES WEDNESDAY
NIGHT SHOULD ALLOW TEMPERATURES TO RADIATE SOME INTO THE UPPER
30S...ESPECIALLY ACROSS NORTH CENTRAL ILLINOIS AND PORTIONS OF THE
FOX RIVER VALLEY...AND LOWER 40S ELSEWHERE. IT APPEARS THAT A
LIGHT NORTHERLY WIND WILL CONTINUE THROUGH MUCH OF THE NIGHT...SO
THE THREAT OF FROST LOOKS LIKE IT WOULD ONLY BE SPOTTY AND
CONFINED TO WELL PROTECTED AREAS...SO WE WILL LEAVE THIS MENTION
OUT OF THE FORECAST FOR NOW. OTHERWISE...EXPECT A SURFACE RIDGE OF
HIGH PRESSURE TO BUILD OVER THE AREA ON THURSDAY. ONSHORE FLOW
WILL AGAIN RESULT IN NEAR 50 DEGREE TEMPERATURES ALONG THE
LAKEFRONT...BUT FARTHER INLAND TEMPERATURES LOOK TO REBOUND BACK
INTO THE LOWER 60S UNDER MAINLY SUNNY SKIES.

MUCH WARMER CONDITIONS ARE EXPECTED ON FRIDAY AS A WESTERLY WIND
SHIFT ALLOWS THE THERMAL RIDGE OVER THE PLAINS TO SHIFT OVER THE
WESTERN GREAT LAKES. MODEL GUIDANCE IS ADVERTISING 925 MB
TEMPERATURES AROUND +20C BY MIDDAY FRIDAY. THIS SHOULD ALLOW
TEMPERATURES TO WARM INTO THE LOWER 80S...ESPECIALLY OVER NORTH
CENTRAL ILLINOIS. THERE IS A POSSIBILITY THAT A LAKE BREEZE COULD
DEVELOP FRIDAY AFTERNOON DUE TO RELATIVELY WEAK WESTERLY FLOW AT
THE LAKE FRONT...AND THIS COULD RESULT IN SLIGHTLY COOLER
CONDITIONS THERE IF ONE DOES DEVELOP. IN ADDITION TO THE QUICK
WARM-UP...I CANNOT TOTALLY RULE OUT A FEW ISOLATED OR WIDELY
SCATTERED THUNDERSTORMS LATE THURSDAY NIGHT INTO FRIDAY WITHIN THE
STRONG WARM AIR ADVECTION REGIME. HOWEVER...DUE TO THE RELATIVELY
WEAK SIGNAL...AND THE LIKELY LOW AREAL COVERAGE...WE HAVE OPTED TO
LEAVE THE FORECAST DRY AT THIS TIME.

A POTENT AND FAST EASTWARD MOVING STORM SYSTEM ACROSS SOUTHERN
CANADA FRIDAY INTO SATURDAY LOOKS TO DROP A COLD FRONT SOUTHWARD
OVER THE AREA BY LATE SATURDAY. THIS SHOULD SET THE STAGE FOR OUR
NEXT DECENT THREAT FOR SHOWERS AND THUNDERSTORMS. ALTHOUGH DEEP
LAYER SHEAR IS NOT ADVERTISED TO BE OVERLY IMPRESSIVE OVER THE
AREA...WE CANT RULE OUT A FEW STRONG THUNDERSTORMS WITH THIS
FRONTAL PASSAGE SATURDAY AFTERNOON. ALSO...WITH THE TIMING OF THIS
FRONT LOOKING TO BE DURING THE MIDDAY TO AFTERNOON HOURS...IT
APPEARS HIGHS ON SATURDAY COULD AGAIN REACH INTO THE 70S IN MOST
INLAND AREAS...TO POSSIBLY LOW 80S AGAIN SOUTH PRIOR TO THE ONSET
OF CONVECTION WITH THE APPROACHING FRONTAL BOUNDARY.

SURFACE HIGH PRESSURE LOOKS TO SET UP OVER THE AREA BY SUNDAY...SO
DRY...BUT COOLER CONDITIONS IN THE 60S ARE EXPECTED.
HOWEVER...WARMER CONDITIONS...INTO THE 70S...BEGIN TO RETURN BY
EARLY NEXT WEEK. IT ALSO APPEARS THE THE WEATHER PATTERN COULD BE
CONDUCIVE TO ADDITIONAL PERIODS OF SHOWERS AND THUNDERSTORMS INTO
NEXT WEEK AS MODEL/ENSEMBLE GUIDANCE ALL ARE FORECASTING ANOTHER
STORM SYSTEM TO TAKE SHAPE OVER THE PLAINS AND THEN SHIFT INTO THE
UPPER MIDWEST BY THE MIDDLE OF NEXT WEEK.

KJB

&&

.AVIATION...
FOR THE 06Z TAFS...

LINE OF SHOWERS AND THUNDERSTORMS IS DROPPING INTO NORTHERN
ILLINOIS LATE THIS EVENING AND SHOULD BE CLEAR OF THE TERMINALS
BY AROUND 7-8Z. MOST LIGHTNING ACTIVITY THUS FAR HAS BEEN FOCUSED
OVER NORTH CENTRAL ILLINOIS WITH ONLY ISOLATED ACTIVITY FARTHER
EAST...SO WILL ONLY BRING THUNDERSTORMS ON STATION AT RFD...AND
WILL JUST KEEP A VCTS MENTION FOR THE CHICAGO AREA TERMINALS. A
GUST FRONT HAS SURGED OUT AHEAD OF THE MAIN LINE OF SHOWERS AND
WILL BRING STRONG GUSTY NORTHERLY WINDS...THEN BEHIND THE INITIAL
LINE OF SHOWERS AND THUNDERSTORMS THERE MAY BE SOME VARIABILITY OR
LIGHTER WINDS BEFORE THE SYNOPTIC FRONT PUSHES THROUGH A FEW HOURS
LATER WITH GUSTY WINDS PICKING UP AGAIN OUT OF THE NORTH.
CONFIDENCE IS HIGH IN THE GUSTY NORTH WINDS WITH THE GUST
FRONT...HOWEVER CONFIDENCE IN WIND DIRECTION IS LOW FOR THE FEW
HOURS FOLLOWING THE LINE OF PRECIP. HAVE BACKED OFF ON THE IFR
CIGS BEHIND THE FRONT THIS EVENING AS THEY ARE NOT SHOWING UP YET
IN UPSTREAM OBS AND MODELS SEEM TO HAVE BETTER CONSENSUS ON MVFR
OR BETTER CONDITIONS. STRONG GUSTY NORTHERLY WINDS ARE EXPECTED TO
CONTINUE THROUGH THE DAY WEDNESDAY WITH SOME SCATTERED DIURNAL
LIGHT SHOWERS POSSIBLE MAINLY DURING THE AFTERNOON.

BMD

&&

.MARINE...
244 AM CDT

A COLD FRONT IS SWEEPING SOUTH ACROSS LAKE MICHIGAN EARLY THIS
MORNING WITH NORTHERLY WINDS INCREASING IN ITS WAKE. THE GRADIENT
WILL BE STRONGEST TODAY AND TONIGHT ACROSS THE SOUTH HALF OF
LAKE MICHIGAN SUPPORTING GALE FORCE WIND GUSTS THROUGH THAT TIME
FRAME. A GALE WARNING IS IN EFFECT FOR THE OPEN WATERS OF LAKE
MICHIGAN SOUTH OF MANISTEE...AND FOR THE INDIANA/ILLINOIS
NEARSHORE WATERS. HIGH PRESSURE WILL BEGIN BUILD ACROSS NORTHERN
LAKE MICHIGAN THURSDAY ALLOWING WINDS TO DIMINISH. THE HIGH WILL
MOVE SOUTH OF THE LAKE LATE THURSDAY INTO FRIDAY AS LOW PRESSURE
MOVES ACROSS MANITOBA INTO ONTARIO WITH A FRESH SOUTHWESTERLY
BREEZE DEVELOPING...THEN A TRAILING COLD FRONT WILL MOVE SOUTH
ACROSS LAKE MICHIGAN ON SATURDAY WITH NORTHERLY WINDS TO 30 KT
EXPECTED BEHIND THE FRONT.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...ILLINOIS NEARSHORE WATERS UNTIL 3 PM WEDNESDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...3 PM WEDNESDAY TO
     10 AM THURSDAY.

     GALE WARNING...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779 UNTIL 4 AM
     THURSDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...4 AM THURSDAY TO 3
     PM THURSDAY.

&&

$$

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